Mollusca

Phylum Mollusca Facts

  • 100,000 species

  • second richest phylum

  • 64% marine, 8% freshwater, 28% land

  • all soft bodied 

  • most secrete a hard shell

  • coelomates

  • trochophore larvae

Members of Mollusca

I. Polyplacophora (chitons)

II. Scaphopoda (toothshells)

III. Gastropoda (snails, slugs, whelks, and limpets)

IV. Bivalvia (clams, mussels, oysters, and scallops)

V. Cephalopoda (squid, cuttlefish, nautili, octopi)

Polyplacophora

  • “many plate bearing”

  • oval-shaped body

    • shell is segmented with 8 overlapping dorsal plates

    • body is un-segmented 

  • use radula to scrape algae off rocks

Scaphopoda

  • “toothshells”

  • tusk-shaped shell

  • opens at both ends

  • burrow in ocean sediment 

  • used by Native Americans as currency

Gastropoda

  • includes ¾ of living mollusks

  • mostly marine

  • some freshwater

  • some terrestrial (snails and slugs)

  • anatomy

    • single, spiraled shell

    • calcium carbonate secreted by mantle

    • what advantage?

      • protection

      • allowed evolutionary increase in visceral mass

    • radula - in the mouth region, a rasp-like feeding organ acting like a backhoe scrapping and scooping using its backwards teeth

    • nephridium - excretory organs, remove metabolic waste

    • heart

    • many have eyes at the tip of tentacles

  • locomotion

    • rippling motion of foot

    • cilia

    • mucus (often leaves a trail)

  • reproduction

    • many are hermaphroditic

    • some are dioecious (2 sexes)

    • internal fertilization

  • foraging

    • most use their radula to graze on plants/algae

    • several predatory species

      • modified radula bores holes in mollusc shells

Slugs
Sea Slugs
Limpet
Cone Snail
Torsion

  • distinctive developmental process

  • as embryo develops, visceral mass rotates

  • anus and mantle cavity wind up over head

  • explanations:

    • creates current for respiration

    • allows retraction of head 

    • reduces amount of sediment in mantle cavity

    • improves center of gravity

  • some species experience detorsion

Bivalvia

  • ecology

    • entirely aquatic

    • sedentary

    • suspension/filter feeders 

    • mussels tether with byssal threads 

    • clams bury in the sediment

  • reproduction

    • mostly dioecious (distinct male and female individuals)

    • most external fertilization

    • some have parasitic “glochidium” larva

  • anomaly

    • shipworms

    • use shell to excavate

    • digest cellulose

    • symbiotic bacteria

      • analogous strategy

  • senses

    • some have eyes

      • scallops

Cephalopoda

  • marine

  • shell is generally reduced

  • active predators

    • adaptations:

      • tentacles to grasp prey

      • biting beak-like jaws

      • poisonous saliva

      • foot has become siphon for locomotion

      • closed circulatory system

      • well-developed brains

Nautilus

  • unique buoyancy control

  • low and flexible metabolic rate

Octopi

  • lack of shell makes them vulnerable 

  • use ink cloud to evade predators

Colossal Squid

  • largest invertebrate

  • weighs half a ton

  • hooks on its tentacles 

  • giant beak

  • never seen in the wild